A power system that supplies power from a battery or other DC power supply to a load, such as a motor, is mainly constituted by a capacitance element, a power switching device, and a power converter. The capacitance element is a smoothing capacitor or the like. The power switching device switches an electric circuit connecting the DC power supply and the capacitance element in response to commands. The power convertor converts DC power to AC power.
While not driving the load, the power switching device leaves the electric circuit connecting the DC power supply and the capacitance element in an open state (i.e. cuts off the electric circuit). Conversely, when driving the load, the power switching device places the electric circuit in a closed state (allows conduction between the power supply and the capacitance element). When placing the electric circuit into the closed state, it is necessary to first charge the capacitance element. Inrush current therefore flows along the electric circuit. Such inrush current may damage elements along the electric circuit between the power supply and the capacitance element, or may adversely affect other devices due to a temporary lowering in the power supply voltage. In order to solve this problem, a variety of power switching devices that use a precharging method are well known (for example, Patent Literature 1 and 2). Such devices restrict the inrush current when the electric circuit enters the closed state by charging the capacitance element via a resistor for a predetermined time period starting at the time a command to place the electric circuit into the closed state is received.
FIG. 34 illustrates the overall configuration of a power system 1000 that includes a power switching device that uses a precharging method according to Patent Literature 1.
The power system 1000 is provided with a power switching device 91, a capacitance element 92, and a power converter 93. The power switching device 91 is provided along an electric circuit connecting a DC power supply BA and the capacitance element 92 and switches the electric circuit in accordance with commands received from an external source. The capacitance element 92 is a smoothing capacitor. The power converter 93 is provided along an electrical circuit connecting the capacitance element 92 and a three-phase AC motor 94 (hereinafter simply referred to as a “motor”). The power converter 93 is an inverter that converts DC to three-phase AC.
In more detail, the power switching device 91 is provided with switches (system main relays SMR1 and SMR2, switching element 98) inserted along the electric circuit connecting the DC power supply BA and the capacitance element 92, a diode 99 for backflow prevention, and a controller 95 for turning the switches on and off. Whether the system main relays SMR1 and SMR2 are on or off is controlled by whether power is supplied to excitation circuits 96 and 97. Whether the switching element 98 is on or off is controlled by a control signal (gate voltage) to the gate terminal thereof. An element with a high on-resistance is used as the switching element 98 (see FIG. 4 of Patent Literature 1).
The controller 95 causes the switches to operate as follows. First, starting at the time a command to place the electric circuit into the closed state is received, the controller 95 turns on the switching element 98 for a predetermined time period, leaving the system main relay SMR1 off. After the predetermined time period has elapsed, the controller 95 turns the switching element 98 off and turns the system main relay SMR1 on. The system main relay SMR2 is on continually during these operations. As a result of these operations, immediately after power is supplied, current flows through the switching element 98, which has a high on-resistance. This prevents inrush current, since the current flowing along the electric circuit connecting the DC power supply BA and smoothing capacitor 92 does not grow excessively large.
[Citation List]
[Patent Literature]
Patent Literature 1: Japanese Patent Application Publication No. 2009-44914
Patent Literature 2: Japanese Patent Application Publication No. 2005-312156